error compiling on VS2022

[chund]

Description

this error:

json.hpp(19176,26): error C2760: syntax error: '&' was unexpected here; expected 'type specifier'

is given on line:

                    new (&*it) value_type{std::move(*next)};

Reproduction steps

compile json.hpp in Visual Studio 2022 latest version.
using C++17
having /sdl on or off does not make a difference.

Expected vs. actual results

compile without error ;)

Minimal code example

No response

Error messages

json.hpp(19176,26): error C2760: syntax error: '&' was unexpected here; expected 'type specifier'

Compiler and operating system

Windows Visual Studio 2022

Library version

3.11.3

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[chund]

related to #4386 ?

[nlohmann]

We use Visual Studio 2022 in our CI and can compile and test the library without issue, see https://github.com/nlohmann/json/blob/develop/.github/workflows/windows.yml#L92. Can you provide more information on what you compile exactly?

[gregmarr]

I'm going to channel Raymond Chen and do some psychic debugging. Because this is on a placement new line, I think that you have something somewhere that redefines new for memory tracking purposes, like below.

#define new DEBUG_NEW
#define DEBUG_NEW new (__FILE__, __LINE__)

This breaks placement new calls because you end up with new (__FILE__, __LINE__) (&*it) value_type{std::move(*next)}; . Your options are to remove that, include the library header before that define, or add protection around the include so it doesn't apply.

#pragma push_macro("new")
#undef new
#include <nlohmann/json.hpp>
#pragma pop_macro("new")

[chund]

This works
(&*it)->operator new(value_type{std::move(*next)});

And yes probably it is because of redefined new. Thanks for that 😀 should have considered that.